HAMR thermal sensor with fast response time

ABSTRACT

Embodiments disclosed herein generally relate to a method for monitoring optical power in a HAMR device. In one embodiment, the method includes enhancing a thermal sensor bandwidth through advanced electrical detection techniques. The advanced electrical detection techniques include obtaining calibration waveform data for a thermal sensor by calibrating the thermal sensor, obtaining real-time waveform data for the thermal sensor that may deviate from the calibration waveform data, updating the calibration waveform data to include the real-time waveform data, repeating obtaining real-time waveform data and updating the calibration waveform data during writing operations. By updating the calibration waveform data, the bandwidth of the thermal sensor is determined by a fixed sampling time interval, and the thermal sensor rise time to steady state would not be a limitation to its response time.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 15/160,325, filed May 20, 2016, which is herein incorporated byreference.

BACKGROUND Field

Embodiments disclosed herein generally relate to a magnetic media deviceemploying a heat-assisted magnetic recording (HAMR) head, and moreparticularly, to a device formed by a method for monitoring opticalpower in the HAMR head.

Description of the Related Art

Higher storage bit densities in magnetic media used in media drives,such as disk drives, have reduced the size (volume) of magnetic bits tothe point where the magnetic bit dimensions are limited by the grainsize of the magnetic material. Although grain size can be reducedfurther, the data stored within the magnetic bits may not be thermallystable. That is, random thermal fluctuations at ambient temperatures maybe sufficient to erase data. This state is described as thesuperparamagnetic limit, which determines the maximum theoreticalstorage density for a given magnetic media. This limit may be raised byincreasing the coercivity of the magnetic media or by lowering thetemperature. Lowering the temperature may not always be practical whendesigning hard disk drives for commercial and consumer use. Raising thecoercivity, on the other hand, requires write heads that incorporatehigher magnetic moment materials, or techniques such as perpendicularrecording (or both).

One additional solution has been proposed, which uses heat to lower theeffective coercivity of a localized region on the magnetic media surfaceand writes data within this heated region. The data state becomes“fixed” once the media cools to ambient temperatures. This technique isbroadly referred to as heat-assisted magnetic recording, or HAMR, whichcan be applied to longitudinal and perpendicular recording systems aswell as “bit patterned media”. Heating of the media surface has beenaccomplished by a number of techniques such as focused laser beams ornear-field optical sources.

The optical power in the light delivery path of HAMR heads affects theheating temperature profile, and hence the recording quality during HAMRrecording. During writing operations, optical power from a laser diode(LD) may fluctuate due to mode hopping, operation temperature drift, andaging. Monitoring and controlling this optical power can improve HAMRrecording quality, reliability and head lifetime. Therefore, an improvedmethod for monitoring optical power in a HAMR device is needed.

SUMMARY

Embodiments disclosed herein generally relate to a device formed by amethod for monitoring optical power in a HAMR device. In one embodiment,the method includes enhancing a thermal sensor bandwidth throughadvanced electrical detection techniques. The advanced electricaldetection techniques include obtaining calibration waveform data for athermal sensor by calibrating the thermal sensor, obtaining real-timewaveform data for the thermal sensor that may deviate from thecalibration waveform data, updating the calibration waveform data toinclude the real-time waveform data, repeating obtaining deviatedwaveform data and updating the calibration waveform data during writingoperations. By updating the calibration waveform data, the bandwidth ofthe thermal sensor is determined by a fixed sampling time interval, andthe thermal sensor rise time to steady state would not be a limitationto its response time.

In one embodiment, a device formed by a method includes obtainingcalibration waveform data for a resistance of a thermal sensor,obtaining real-time waveform data for the resistance of the thermalsensor, updating the calibration waveform data to include the real-timewaveform data, and repeating the obtaining real-time waveform data andupdating the calibration waveform data.

In another embodiment, a device formed by a method includes heating athermal sensor to a predetermined temperature, maintaining thetemperature of the thermal sensor at the predetermined temperature whilethe thermal sensor is operating at a steady state, measuring andtracking a resistance value of the thermal sensor, and maintaining theresistance value of the thermal sensor at a substantially constantvalue.

In another embodiment, a device formed by a method includes heating athermal sensor to a predetermined temperature, obtaining calibrationwaveform data for a resistance of the thermal sensor, maintaining thetemperature of the thermal sensor at the predetermined temperature whilethe thermal sensor is operating at a steady state, measuring andtracking a resistance value of the thermal sensor, maintaining theresistance value of the thermal sensor at a substantially constantvalue, obtaining real-time waveform data for the resistance of thethermal sensor, updating the calibration waveform data to include thereal-time waveform data, and repeating the obtaining real-time waveformdata and updating the calibration waveform data.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the disclosurecan be understood in detail, a more particular description of thedisclosure, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this disclosure and are therefore not to beconsidered limiting of its scope, for the disclosure may admit to otherequally effective embodiments in any field involving magnetic sensors.

FIG. 1A illustrates a magnetic media drive system according to oneembodiment described herein.

FIG. 1B is a cross sectional schematic view of a HAMR enabled write headaccording to one embodiment described herein.

FIG. 2 illustrates a method for increasing bandwidth of a thermal sensorshown in FIG. 1B according to one embodiment described herein.

FIG. 3A illustrates an example of calibration waveform data for thethermal sensor shown in FIG. 1B according to one embodiment describedherein.

FIG. 3B illustrates an example of updated calibration waveform data forthe thermal sensor shown in FIG. 1B according to one embodimentdescribed herein.

FIG. 4 illustrates a method for increasing bandwidth of the thermalsensor shown in FIG. 1B according to another embodiment describedherein.

FIG. 5 illustrates a method for increasing bandwidth of the thermalsensor shown in FIG. 1B according to another embodiment describedherein.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. It is contemplated that elements disclosed in oneembodiment may be beneficially utilized on other embodiments withoutspecific recitation.

DETAILED DESCRIPTION

In the following, reference is made to embodiments. However, it shouldbe understood that the disclosure is not limited to specific describedembodiments. Instead, any combination of the following features andelements, whether related to different embodiments or not, iscontemplated to implement and practice the claimed subject matter.Furthermore, although embodiments described herein may achieveadvantages over other possible solutions and/or over the prior art,whether or not a particular advantage is achieved by a given embodimentis not limiting of the claimed subject matter. Thus, the followingaspects, features, embodiments and advantages are merely illustrativeand are not considered elements or limitations of the appended claimsexcept where explicitly recited in a claim(s).

Embodiments disclosed herein generally relate to a device formed by amethod for monitoring optical power in a HAMR device. In one embodiment,the method includes enhancing a thermal sensor bandwidth throughadvanced electrical detection techniques. The advanced electricaldetection techniques include obtaining calibration waveform data for athermal sensor by calibrating the thermal sensor, obtaining real-timewaveform data for the thermal sensor that may deviate from thecalibration waveform data, updating the calibration waveform data toinclude the real-time waveform data, repeating obtaining real-timewaveform data and updating the calibration waveform data during writingoperations. By updating the calibration waveform data, the bandwidth ofthe thermal sensor is determined by a fixed sampling time interval, andthe thermal sensor rise time to steady state would not be a limitationto its response time.

FIG. 1A illustrates a magnetic media drive 100 embodying thisdisclosure. As shown, at least one magnetic medium 112, such as amagnetic disk, is supported on a spindle 114 and rotated by a mediadrive motor 118. The magnetic recording on each medium is in the form ofany suitable patterns of data tracks, such as annular patterns ofconcentric data tracks (not shown) on the magnetic medium 112.

At least one slider 113 may be positioned near the magnetic medium 112,each slider 113 supporting one or more magnetic head assemblies 121 thatmay include a radiation source (e.g., a LD or LED) for heating themedium surface 122. As the magnetic medium 112 rotates, the slider 113moves radially in and out over the medium surface 122 so that themagnetic head assemblies 121 may access different tracks of the magneticmedium 112 to read or record data. Each slider 113 is attached to anactuator arm 119 by way of a suspension 115. The suspension 115 providesa slight spring force which biases the slider 113 toward the mediumsurface 122. Each actuator arm 119 is attached to an actuator means 127.The actuator means 127 as shown in FIG. 1A may be a voice coil motor(VCM). The VCM includes a coil movable within a fixed magnetic field,the direction and speed of the coil movements being controlled by themotor current signals supplied by control unit 129.

During operation of a HAMR enabled magnetic media drive 100, therotation of the magnetic medium 112 generates an air bearing between theslider 113 and the medium surface 122 which exerts an upward force orlift on the slider 113. The air bearing thus counter-balances the slightspring force of suspension 115 and supports slider 113 slightly abovethe medium surface 122 by a small, substantially constant spacing duringnormal operation. The radiation source heats up the high-coercivitymedia so that the write elements of the magnetic head assemblies 121 maycorrectly magnetize the data bits in the magnetic medium 112.

The various components of the magnetic media drive 100 are controlled inoperation by control signals generated by control unit 129, such asaccess control signals and internal clock signals. Typically, thecontrol unit 129 comprises logic control circuits, storage means and amicroprocessor. The control unit 129 generates control signals tocontrol various system operations such as drive motor control signals online 123 and head position and seek control signals on line 128. Thecontrol signals on line 128 provide the desired current profiles tooptimally move and position slider 113 to the desired data track onmedia 112. Write and read signals are communicated to and from write andread heads on the magnetic head assemblies 121 by way of recordingchannel 125.

The above description of a typical magnetic media storage system and theaccompanying illustration of FIG. 1A are for representation purposesonly. It should be apparent that magnetic media storage systems maycontain a large number of media and actuators, and each actuator maysupport a number of sliders.

FIG. 1B is a cross sectional schematic view of a HAMR enabled write head101, according to one embodiment described herein. The head 101 isoperatively attached to a laser diode (LD) 155 that is powered by alaser driver 150. The LD 155 may be placed directly on the head 101 orradiation may be delivered from the LD 155 located off the slider 113through an optical fiber or waveguide 135. Similarly, the laser driver150 circuitry may be located on the slider 113 or on a system-on-chip(SOC) associated with the magnetic media drive 100 such as control unit129. The head 101 includes a spot-size converter 130 for focusing theradiation transmitted by the LD 155 into the waveguide 135. In anotherembodiment, the magnetic media drive 100 may include one or more lensfor focusing the beamspot of the LD 155 before the emitted radiationreaches the spot-size converter 130. The waveguide 135 is a channel thattransmits the radiation through the height of the head 101 to a nearfield transducer 140—e.g., a plasmonic device or an opticaltransducer—which is located at or near a media facing surface (MFS) 144,such as an air bearing surface (ABS). The waveguide 135 may extend in adirection that is substantially perpendicular to the MFS 144, as shownin FIG. 1B. Alternatively, the waveguide 135 may not extend in adirection that is substantially perpendicular to the MFS 144, due toturns in the plane of the substrate or for better coupling to the NFT140. The NFT 140 further focuses the beamspot to avoid heatingneighboring tracks of data on the medium 112—i.e., creates a beamspotmuch smaller than the diffraction limit. As shown by arrows 142, thisoptical energy emits from the NFT 140 to the surface of the medium 112below the MFS 144 of the head 101. The embodiments herein are notlimited to any particular type of NFT and may operate with, for example,either a c-aperature, e-antenna plasmonic near-field source, or anyother shaped transducer known in the art.

A thermal sensor 145, or a waveguide sensor, is located adjacent thewaveguide 135. The thermal sensor 145 may be a thermistor or resistancetemperature detector (RTD) where the electrical resistance of thematerial comprising the thermal sensor 145 changes as the temperature ofthe material varies (either inversely or directly). As the light from LD155 goes through the waveguide 135, about five percent or less of theoptical power is used to heat the thermal sensor 145, causing theresistance of the thermal sensor 145 to change. The thermal sensor 145may be electrically coupled to the laser driver 150 or some othercontrol device to measure the electrical resistance of the thermalsensor 145. In one embodiment, the thermal sensor 145 is electricallycoupled to a preamplifier 160 attached to a head stack assembly (notshown). This change in electrical resistance in the thermal sensor 145may then be used as a feedback control signal to adjust the currentsupply of the LD 155.

A second thermal sensor 147, or a reference sensor, may be locatednearby the thermal sensor 145 so the two sensors are at a similarbackground temperature. However, the second thermal sensor 147 may befarther away from the waveguide 135 than the thermal sensor 145, thusthe second thermal sensor 147 is illustrated in dotted line since thesecond thermal sensor 147 may not be viewable from the cross sectionaldiagram shown in FIG. 1B. During writing with the LD 155, the differencein the resistance between the thermal sensor 145 and the thermal sensor147 is measured. With the thermal sensor 145 and the thermal sensor 147,monitoring the optical power may be achieved while no optical componentsare needed, no additional assembly steps are needed, and no significantreflection is added to the system which can create adverse interferenceeffects due to the coherency of the laser light. In addition, with twothermal sensors, temperature variations due to optical power fluctuationor ambient temperature drift may be differentiated.

Typically, the thermal sensor 145 has a relatively slow response time,and it is very difficult for the bandwidth of the thermal sensor 145 tobe above 1 MHz. In order to increase the bandwidth of the thermal sensor145, advanced electrical detection techniques are utilized. FIG. 2illustrates a method 200 for increasing the bandwidth of the thermalsensor 145 according to one embodiment described herein. The method 200starts with obtaining calibration waveform data for the resistance of athermal sensor, such as the thermal sensor 145, by calibrating thethermal sensor, as shown at block 202. The calibration of the thermalsensor is conducted when the thermal sensor is at a transient state,i.e., before reaching a steady state, and the transient waveform of thethermal sensor is intrinsic to the thermal sensor.

FIG. 3A illustrates an example of calibration waveform data for thethermal sensor according to one embodiment described herein. As shown inFIG. 3A, calibration waveform data 302 of a response time for thethermal sensor is shown as a relationship between sensor resistance andtime. The calibration waveform data 302 is obtained while the thermalsensor is operating at the transient state before reaching a steadystate. As shown in FIG. 3A, at around 100 nanoseconds (ns), a LD, suchas the LD 155, is turned on, and the resistance of the thermal sensorbefore turning on the LD stays constant. After the LD is turned on, theresistance of the thermal sensor increases due to heating from theevanescent field of a waveguide, such as the waveguide 135 shown in FIG.1B. The calibration waveform data 302 is obtained without the LD havingany power fluctuation. Different fluctuation magnitudes can lead to adifferent scale of the waveform data.

Referring back to FIG. 2, at block 204, real-time waveform data for theresistance of the thermal sensor that may deviate from the calibrationwaveform data is obtained by sampling the thermal sensor resistance at apredetermined time interval. The deviation of the real-time waveformdata for the thermal sensor may be greater than noise in the waveformdata and may be greater than a predetermined threshold value. In oneembodiment, the threshold value is at most one percent deviation fromthe sensor resistance in the calibration waveform data. Once thedeviation in the real-time waveform data is determined not to be noise,the deviation in the real-time waveform data can be presumed to becaused by a power fluctuation in the LD. The power fluctuation in the LDmay be caused by mode hopping, operation temperature drift, and aging.The predetermined sampling time interval can be set at 10 ns,corresponding to a bandwidth of 100 MHz, which is much higher than thetypical bandwidth of the thermal sensor. The predetermined sampling timeinterval may range from about 5 ns to about 20 ns. The bandwidth of thethermal sensor is based on the sampling time interval, and the typicalresponse time of the thermal sensor is not a limitation to the operationbandwidth of the thermal sensor.

Next, at block 206, the calibration waveform data is updated to includethe real-time waveform data obtained at block 204. FIG. 3B illustratesan example of updated calibration waveform data for the thermal sensoraccording to one embodiment described herein. As shown in FIG. 3B, theupdated calibration waveform data includes the calibration waveform data302 and real-time waveform data 304 for the thermal sensor. Thereal-time waveform data 304 is registered at about 250 ns when thesensor resistance of the thermal sensor is deviated from the calibrationwaveform data by at most one percent. The real-time waveform data 304becomes the standard, or the anticipated waveform data, for anysubsequent real-time waveform data to compare thereto.

Referring back to FIG. 2, at block 208, the obtaining real-time waveformdata (block 204) and updating the calibration waveform data (block 206)are repeated whenever laser diode is turned on (write operation occurs).Any deviated real-time waveform data obtained can be used to presumethat there is a power fluctuation in the LD, and actions, such asstopping the writing process, may be performed in order to compensatefor the power fluctuation. The deviated real-time waveform data is addedto the calibration waveform data each time the deviated real-timewaveform data is obtained, and the current deviated real-time waveformdata serves as the standard for the next deviated real-time waveformdata to compare to. The thermal sensor's bandwidth is based on thesampling time interval, which is much faster than the bandwidth based onthe thermal sensor's typical response time.

The method 200 illustrates a way to increase the bandwidth of thethermal sensor by advanced electrical detection techniques, which relateto signal processing techniques. An alternative or additional method toincrease the bandwidth of the thermal sensor is to heat and maintain thethermal sensor at an elevated temperature during operation. Typically,multiple time constants are associated with the transient behavior ofthe thermal sensor. The fast time constant may be as fast as 20 ns,while the slow time constant is around 200 ns to 500 ns. The slow timeconstant may dictate the overall detection bandwidth for the thermalsensor when the thermal sensor is not operating at a steady state.Conventionally, the temperature of the thermal sensor operates at around20 to 25 degrees Celsius and may change as the optical power goingthrough the waveguide changes. Any changes to the temperature of thethermal sensor can affect the operation of the thermal sensor, causingthe thermal sensor not operating at the steady state. As a result, theslow time constant dictate the overall detection bandwidth for thethermal sensor. By heating and maintaining the temperature of thethermal sensor, the thermal sensor's steady state operation is notchanged and the fast time constant dictates the overall detectionbandwidth for the thermal sensor.

FIG. 4 illustrates a method 400 for increasing the bandwidth of thethermal sensor according to one embodiment described herein. The method400 starts with heating a thermal sensor, such as the thermal sensor 145shown in FIG. 1B, to a predetermined temperature, as shown at block 402.The predetermined temperature may range from about 30 degrees Celsius toabout 40 degrees Celsius. The heating of the thermal sensor may beperformed by resistive heating, such as increasing the current flowingthrough the thermal sensor. Next, at block 404, the temperature of thethermal sensor is maintained at the predetermined temperature while thethermal sensor is operating at a steady state prior to a writeoperation. At block 406, the resistance value of the thermal sensor ismeasured and tracked. The resistance value of a reference sensor, suchas the thermal sensor 147 shown in FIG. 1B, may be tracked along withthe resistance value of the thermal sensor. The measuring and trackingof the resistance value of the thermal sensor and the reference sensormay be performed by a preamplifier, such as the preamplifier 160 shownin FIG. 1B.

Next, at block 408, the resistance of the thermal sensor is maintainedat a substantially constant value during the writing operation. Upon anoptical power fluctuation, the thermal sensor is getting heated orcooled and the resistance value of the thermal sensor starts to rise ordrop with the fast time constant, since the thermal sensor is operatingat a steady state (the temperature of the thermal sensor has not yetchanged enough to cause the thermal sensor to not operate at a steadystate). The heating or cooling of the thermal sensor can be detected bythe preamplifier. The preamplifier reduces or increases the currentflowing through the thermal sensor to compensate for the heating orcooling as a result of the optical power fluctuation, and thepreamplifier deduces the amount of optical power change amplitudethrough the amount of change in the current flowing through the thermalsensor.

By adjusting the current flowing through the thermal sensor (increasingor decreasing), the temperature of the thermal sensor and the resistanceof the thermal sensor are kept at a substantially constant level, andthe thermal sensor's steady state operation is maintained. Since thethermal sensor is operating at a steady state, any optical powerfluctuation can be detected by the thermal sensor with the fast timeconstant, which increases the bandwidth of the thermal sensor.

The method 400 described herein may be performed along with the method200 described herein in order to further increase the bandwidth of thethermal sensor. FIG. 5 illustrates a method 500 for increasing thebandwidth of the thermal sensor shown in FIG. 1B according to anotherembodiment described herein. The method 500 is a method combining themethod 200 and the method 400. The method 500 starts with heating athermal sensor to a predetermined temperature, as shown at block 502.The method shown at block 502 may be the same as the method shown atblock 402. Next, calibration waveform data for the resistance of thethermal sensor, such as the thermal sensor 145, is obtained bycalibrating the thermal sensor, as shown at block 504. The method shownat block 504 may be the same as the method shown at block 202. Next,real-time waveform data for the resistance of the thermal sensor thatmay deviate from the calibration waveform data is obtained by samplingthe thermal sensor resistance at a predetermined time interval, as shownat block 506. The method shown at block 506 may be the same as themethod shown at block 204. Next, at block 508, the calibration waveformdata is updated to include the real-time waveform data obtained at block506. The method shown at block 508 may be the same as the method shownat block 206. Next, at block 510, the obtaining real-time waveform data(block 506) and updating the calibration waveform data (block 508) arerepeated during writing operations. The method shown at block 510 may bethe same as the method shown at block 208.

Next, at block 512, the temperature of the thermal sensor is maintainedat the predetermined temperature while the thermal sensor is operatingat a steady state. The method shown at block 512 may be the same as themethod shown at block 404. At block 514, the resistance value of thethermal sensor is measured and tracked. The method shown at block 514may be the same as the method shown at block 406. Next, at block 516,the resistance of the thermal sensor is maintained at a substantiallyconstant value during the writing operation. The method shown at block516 may be the same as the method shown at block 408. At block 516, thethreshold resistance value change as described at block 506 may be verysmall, such that the change in the resistance value is small enough thatthe resistance is still considered as substantially constant asdescribed at block 516. By combining these two approaches, the bandwidthof the sensor is no longer limited by the fast time constant of itstransient response.

In summary, methods for decreasing response time of a thermal sensor ina HAMR device are disclosed. In one embodiment, the response time of thethermal sensor is decreased by obtaining calibration waveform data andupdating the calibration waveform data to include deviated waveformdata. The sampling time interval use to sample waveform data dictatesthe operation bandwidth of the thermal sensor instead of the responsetime based on the thermal sensor's typical response time. In anotherembodiment, the response time of the thermal sensor is decreased byheating and maintaining the thermal sensor at an elevated temperature.Maintaining the temperature of the thermal sensor at a substantiallyconstant level leads to operating the thermal sensor at a steady state,and the fast time constant of the thermal sensor dictates the overalldetection bandwidth of the thermal sensor. The methods described abovemay be combined to further decrease the response time of the thermalsensor.

While the foregoing is directed to embodiments of the disclosure, otherand further embodiments may be devised without departing from the basicscope thereof, and the scope thereof is determined by the claims thatfollow.

What is claimed is:
 1. A device formed by a method, comprising:obtaining calibration waveform data for a resistance of a thermalsensor; measuring real-time waveform data for the resistance of thethermal sensor by sampling the resistance of the thermal sensor at apredetermined time interval, wherein the predetermined time intervalcorresponds to a bandwidth greater than a bandwidth of the thermalsensor; determining deviated real-time waveform data; updating thecalibration waveform data to include the deviated real-time waveformdata; and repeating the measuring real-time waveform data, determiningdeviated real-time waveform data and updating the calibration waveformdata.
 2. The device of claim 1, wherein the deviated real-time waveformdata is greater than a predetermined threshold value.
 3. The device ofclaim 2, wherein the predetermined threshold value is at most onepercent deviation from a resistance in the calibration waveform data. 4.The device of claim 1, wherein the predetermined time interval rangesfrom about 5 nano seconds to about 20 nano seconds.
 5. The device ofclaim 1, wherein the deviated real-time waveform data is caused by apower fluctuation in a laser diode.
 6. A device formed by a method,comprising: heating a thermal sensor to a predetermined temperaturegreater than an operating temperature of the thermal sensor; maintainingthe temperature of the thermal sensor at the predetermined temperaturewhile the thermal sensor is operating at a steady state; measuring andtracking a resistance value of the thermal sensor; and maintaining theresistance value of the thermal sensor at a substantially constantvalue.
 7. The device of claim 6, wherein the predetermined temperatureranges from about 30 degrees Celsius to about 40 degrees Celsius.
 8. Thedevice of claim 6, wherein the heating of the thermal sensor isperformed by resistive heating.
 9. The device of claim 6, furthercomprising measuring and tracking a resistance of a reference sensor.10. The device of claim 6, wherein the maintaining the resistance valueof the thermal sensor at the substantially constant value comprisesincreasing or decreasing a current flowing through the thermal sensor.11. The device of claim 10, wherein the increasing or decreasing thecurrent flowing through the thermal sensor is performed by apreamplifier.
 12. A device formed by a method, comprising: heating athermal sensor to a predetermined temperature greater than an operatingtemperature of the thermal sensor; obtaining calibration waveform datafor a resistance of the thermal sensor; obtaining real-time waveformdata for the resistance of the thermal sensor; updating the calibrationwaveform data to include the real-time waveform data; repeating theobtaining real-time waveform data and updating the calibration waveformdata; maintaining the temperature of the thermal sensor at thepredetermined temperature while the thermal sensor is operating at asteady state; measuring and tracking a resistance value of the thermalsensor; and maintaining the resistance value of the thermal sensor at asubstantially constant value.
 13. The device of claim 12, wherein adeviation of the real-time waveform data from the calibration waveformdata is greater than a predetermined threshold value.
 14. The device ofclaim 13, wherein the predetermined threshold value is at most onepercent deviation from a resistance shown in the calibration waveformdata.
 15. The device of claim 12, wherein the real-time waveform data isobtained by sampling the resistance of the thermal sensor at apredetermined time interval.
 16. The device of claim 15, wherein thepredetermined time interval ranges from about 5 nano seconds to about 20nano seconds.
 17. The device of claim 12, wherein the predeterminedtemperature ranges from about 30 degrees Celsius to about 40 degreesCelsius.
 18. The device of claim 12, wherein the heating of the thermalsensor is performed by resistive heating.
 19. The device of claim 12,wherein the maintaining the resistance value of the thermal sensor atthe substantially constant value comprises increasing or decreasing acurrent flowing through the thermal sensor.